The present invention relates to a process for the extraction of U (VI) ions and/or Pu (IV) ions present in an acid aqueous solution by means of N,N-dialkylamides.
It more particularly applies to the recovery of uranium and optionally plutonium present in nitric dissolving solutions obtained during the first stage of reprocessing irradiated nuclear fuels.
One of the conventional procedures for reprocessing irradiated nuclear fuels is to firstly dissolve the fuel material in a nitric solution, followed by the treatment of said solution by extraction using an organic solvent for separating the uranium and plutonium from the fission products and then separating the uranium from the plutonium.
For the first extraction operation, use is generally made of an organic solvent constituted by tributyl phosphate and the uranium is then separated from the plutonium extracted in the organic solvent by contacting the latter with a nitric aqueous solution containing reducing agents in order to reduce the plutonium (IV) into plutonium (III) and to pass it into the aqueous solution. This process which is widely used on an industrial scale suffers from certain disadvantages, which are in particular linked with the use of tributyl phosphate as the extractant.
Thus, the chemical and radiolytic degradation products of tributyl phosphate are disturbing, because they give insoluble salts with most of the ions present in the reprocessing solutions, e.g. with U (VI), U (IV), Pu (IV), Pu (III), Zr (IV) and (III).
Moreover, the spent tributyl phosphate-based organic solvents cannot easily be processed by incinerating, because their incineration products are not solely constituted by volatile compounds.
The need to use reducing agents for bringing the plutonium to valency (III) with a view to the uranium-plutonium separation causes new problems which are difficult to solve.
Thus, the conventionally used reducing agents such as valency (II) iron, valency (IV) uranium or hydroxylamine (NH.sub.2 OH) are not stable in the presence of nitric acid because they have a tendency to be oxidized by it. To prevent this, an anti-nitrite agent, such as hydrazine (NH.sub.2 --NH.sub.2) is added to the nitric solution.
However, the use of such reagents is seriously disturbed when the nitric solution also contains technetium, which is generally the case, because the tributyl phosphate also extracts a significant proportion of the technetium present in the irradiated nuclear fuel dissolving solutions. However, the presence of technetium catalyses the oxidation of the hydrazine by nitric acid, thus preventing the hydrazine from playing its part as the stabilizer of the reducing agents of Pu (IV). Therefore there is also an oxidation of the reducing agents, thus preventing the reduction of the plutonium and therefore its reextraction in the aqueous solution.
To obviate this disadvantage, it has been envisaged to increase the added hydrazine quantity, which naturally leads to an increase in the hydrazine decomposition products, such as hydrazoic acid, its salts and ammonium ions, which are undesirable products.
Thus, it is known that hydrazoic acid, which is very volatile and extractable in organic solvents, is liable to give unstable or explosive salts and this applies to a lesser extent for ammoniacal compounds. It is obviously undesirable to increase the explosion risks in an installation for the reprocessing of irradiated nuclear fuels.
It would also be of great interest to obviate the use of reducing agents and hydrazine in processes of this type, particularly as the Pu.sup.3+ ions in the presence of NO.sub.3.sup.- ions are out of equilibrium, because E.sub.app.sup.o Pu.sup.4+ /Pu.sup.3+ is substantially 0.92 V, whereas E.sub.app.sup.o NO.sub.3.sup.- /HNO.sub.2 is substantially 0.94 V in a HNO.sub.3 medium at 1 mol.l.sup.-1.
In addition, for some years now research has been carried out on the possibility of using organic extractants other than tributyl phosphate, in order to obviate these various disadvantages.
Thus, the use of N,N-dialkylamides has been envisaged and research carried out by the Oak-Ridge, Savannah River and Comitato Nazionale per l'Energia Nucleare laboratories have demonstrated that such extractants would have a good affinity for hexavalent and tetravalent ions of actinides, a low affinity for the main fission products, a good resistance to radiolysis and chemical degradation and a limited solubility in aqueous solution, whilst being easy to synthesize and purify, as described by T. H. Siddall in December 1960 in J. Phys. Chem., vol. 64, pp 1863-1866 and by G. M. Gasparini et al in Separation Science and Technology, 15 (4), pp 825-844, 1980.
However, it has not hitherto been possible to use these extractants because the tested uranyl nitrate and N,N-dialkylamide complexes tested have a limited solubility in organic solvents, particularly when using alkanes as the diluents for N,N-dialkylamides. In addition, the use of such extractants has not been considered on an industrial scale.